Mobile phones enable their users to conduct conversations in many different acoustic environments. Some of these are relatively quiet while others are quite noisy. There may be high background or ambient noise, for instance, on a busy street or near an airport or train station. There are also different types of background noise, such as ocean waves, automobile drive-by noise, babble noise (e.g., in a pub), and engine noise, to name just a few. To improve the intelligibility of the near-end user's speech, to a far-end user during a call, an audio signal processing technique known as noise suppression can be implemented in the near-end user's mobile phone. During the mobile phone call, the noise suppressor operates in real-time upon a so-called uplink signal that contains not just speech of the near-end user but also background noise that has been picked up by a primary or voice dominant acoustic microphone (sometimes referred to as the bottom acoustic microphone of a smart phone handset). Before the uplink signal is transmitted by the mobile phone to the communications network (and then onward to the far-end user's device) the noise suppressor attempts to reduce the amount of the background noise that has been picked up by the bottom microphone, by performing noise removal digital signal processing operations upon the uplink signal. These operations rely on what is hopefully an accurate estimate of the background noise.
It is often difficult to discriminate between noise and speech, both of which are present in the same audio signal. The noise estimate or noise profile is often computed as a power or energy spectrum (frequency domain), and may be updated or re-computed for each frame (discrete-time sequence portion) of the uplink signal. There are various known techniques for audio noise estimation. For example, a secondary acoustic microphone may be provided in the handset and that is positioned away from the bottom microphone—this is sometimes referred to as a “top” microphone or a noise dominant microphone. It may be expected that this secondary microphone, due to its orientation and position, should pick up primarily the ambient sound, rather than the near-end user's speech. Signal processing operations are then performed upon the primary and secondary microphone signals to generate a noise profile that in many instances has proven to be more accurate than using just the bottom microphone (to discriminate between speech and noise.)